Among the molded products, prepared on injecting a molding material, such as molten resin, charged into a cavity of a forming metal mold, and subsequently on allowing the so charged molten resin to be cooled and solidified, there are those the surfaces of which are decoratively worked for affording the feeling of the high grade product so that the finished articles appear as if they are formed of a metallic material. Examples of the decorative machining include mirror surface finishing, which works the product surface to a smooth and lustrous finish, a so-called spin mark machining, which forms a large number of fine parallel concentric lines on the product surface, and a so-called hairline machining, which forms a large number of fine parallel straight lines on the product surface.
The molded products with decorative surface machining are mostly formed using a metal mold provided with a transfer surface prepared by electroplating. The conventional manufacturing method for a forming metal mold, used for producing a molded article with such decorative machining, is shown in FIGS. 1 to 4.
In a conventional manufacturing method, a master 101 is first prepared, as shown in FIG. 1. This master 1 is formed of a metal material, such as brass or beryllium copper, and has a diamond cut surface 102 formed on a rim portion of its upper surface by diamond cutting. The upper surface of the master 101 is formed with a plural number of fine concentric scores for forming spin marks on the molded product to provide a spin forming surface 103.
A plating layer 104 then is formed by electroplating on the entire surface of the master 101, as shown in FIG. 2. The plating layer 104 is formed of, for example, chromium, to a thickness of 5 to 6 mm.
The master 101 and the plating layer 104 then are separated from each other to form a recess for filling 105 on the plating layer 104, as shown in FIG. 3. By separating the master 101 and the plating layer 104 from each other, the diamond cut surface 102 and the spin forming surface 103 are transcribed on corresponding surfaces of the plating layer defining the recess for filling 105. The surface to which the diamond cut surface 102 has been transcribed is formed as a mirror surface processing surface 106 for transcription to the surface of a molded article for providing mirror surface finishing to the molded article, whilst the surface to which the spin forming surface 103 has been transcribed is formed as a spin processing surface 107 for transcription to the surface of the molded product to produce spin marks thereon.
The plating layer 104 is inserted into and fixed in an insertion recess 109 of a base mold of the forming mold 108, as shown in FIG. 4. The base mold 108, with the plating layer 104 secured thereto, is set and assembled with a fixed forming mold, not shown, to produce a metallic forming mold. A molding material is charged into the recess for filling 105. When the molding material is cooled and cured, the metal mold is opened to take out the molded product from the recess for filling 105.
On the molded article, thus prepared, there are transcribed the mirror surface processing surface 106 and the spin processing surface 107, such that the molded article appears as if it is molded of a metal material.
With the conventional method, described above, the master 101 is prepared of a metal material, such as brass or beryllium copper, and the plating layer 104 is formed on its surface by electroplating, in order to apply decorative processing to the surface of a molded article.
Usually, the plating layer 104 can be formed to a preset thickness by electroplating only after a processing time interval as long as approximately two months, thus raising an inconvenience of considerably protracted manufacturing time.
Apart from the problem of the protracted manufacturing time, the plating layer 104 needs to be formed in addition to the master 101, thus raising another inconvenience of increased production cost.